add docker building and pushing steps

.github/workflows/docker-build.yaml

@@ -8,10 +8,30 @@ on:
       - closed
 
 jobs:
-  if_merged:
+  build-and-push:
     if: github.event.pull_request.merged == true
     runs-on: ubuntu-latest
     steps:
-    - run: |
-        echo The PR was merged
+      - name: Getting the codebase
+        uses: actions/checkout@v4
+        with:
+          ref: main
+
+      - name: Login to DockerHub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ vars.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Push to hub
+        uses: docker/build-push-action@v6
+        with:
+          push: true
+          tags: ${{ vars.DOCKERHUB_USERNAME }}/biodocker:latest